Some Geological and Environmental Aspects of the Sàrrabus-Gerrei Region (SE Sardinia, Italy)

Rendiconti Seminario Facoltà Scienze Università Cagliari Supplemento Vol. 71 Fasc. 2 (2001)

Some geological and environmental aspects of the Sàrrabus-Gerrei Region (SE Sardinia, Italy)

FRANCESCO SECCHI(*), MARIO LORRAI(**)

Abstract. This paper focus on some geological and environmental aspects of the Sàrrabus-Gerrei region (SE Sardinia, Italy). The first part (leader: Francesco Secchi) is dedicated to the Sàrrabus igneous massif that outcrops over an area of about 400 km2 and is a shallow calcalkaline pluton characterized by multiple intrusions emplaced in the frontal part of a nappe pile at the end of hercynian orogeny. The second part of the trip (leader: Mario Lorrai) is dedicated to the abandoned antimony mine of Su Suergiu (Villasalto, Gerrei) where an important metallurgical activity has been joined with mining works since 1882. In this area, the hydrogeochemical characterization of both ground and surface waters provide useful information about the impact of the past mining activity on the environment.

Riassunto. In questo lavoro vengono presentati alcuni aspetti geologici e ambientali della regione del Sàrrabus-Gerrei. In particolare, la prima parte dell’escursione (leader: Francesco Secchi) offre una panoramica delle caratteristiche del complesso plutonico del Sàrrabus (Sardegna sud-orientale).Esso affiora su un’area di circa 400 km2 ed è un plutone articolato in una serie di unità post-tettoniche messesi in posto a livelli alto-crostali nella parte frontale della catena ercinica della Sardegna. La seconda parte dell’escursione (leader: Mario Lorrai) è dedicata alla visita della miniera abbandonata di Su Suergiu (Villasalto, Gerrei) dove sin dal 1882 all’estrazione di minerali di antimonio è stata affiancata una importante attività metallurgica. In questa area la caratterizzazione geochimica delle acque sotterranee e superficiali fornisce utili informazioni sull’impatto delle passate attività minerarie sull’ambiente.

INTRODUCTION Although in Sardinia pre-hercynian magmatism and deformations were recognized, the structure of the Island is essentially related to the hercynian cycle. At the present time, however, most of unsolved problems regard mainly the granitoids, because of the necessity to mapping large portion of the Sardinia batholith.

(*) Ist. Scienze Geol. Mineral. Corso Angioj 10, 07100, Sassari (Italy).e-mail: [email protected] (**) Dip. Scienze della Terra, Via Trentino 51, 09127 Cagliari (Italy). e-mail: [email protected] 188 F. SECCHI, M. LORRAI

Figure 1. Simplified geological sketch map of late-hercynian granitoids of Sardinia. Legend: (1) undifferentiated post-Palaeozoic sedimentary and volcanic covers; (2) late Palaeozoic volcanic and sedimentary covers; 3-5: late-hercynian granitoids. (3) meta (a) and peraluminous (b) leucogranites; (4) monzogranitic granodiorites and monzogranites; (5) granodiorites; 6-8: metamorphic basement. (6) low-grade metasedimentary and metavolcanic sequences of more external zone; (7) low to medium grade metasedimentary and metavolcanic sequences of Çnappe zoneÈ; (8) high grade metamorphic complex (paragneisses, micaschist and quartzites) with migmatites of axial zone. Other symbols: main overthrusts (9); Posada-Asinara line (10); main regional faults (11). After [24], simplified and modified. Note that in order to simplify late-hercynian gabbrotonalitic masses were not mapped. SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 189

The occurrence of almost all igneous terms of Sardinia batholith, grouped as in the Sàrrabus area in a restricted pluton characterized by good and continuous exposures, offers a good opportunity to focus on some essential problems of plutonic magmatism in orogenic environment. Particularly, will be discussed the geologic significance of the gabbro-tonalitic masses associated with granodioritic sequences, which is one of the most controversial aspects of the orogenic plutonism. The Sàrrabus-Gerrei region has been an important mining district in Sardinia and the environmental consequences of these activities are still detectable in many areas. The main results of hydrogeochemical investigations in the area of the abandoned mine of Su Suergiu (Villasalto, Gerrei) are discussed, pointing out the mobilization of contaminants (mainly arsenic and antimony) in the environment.

THE PALAEOZOIC BASEMENT OF SARDINIA The metamorphic basement The Sardinia Palaeozoic basement represents a fragment of the South European hercynian chain originated by the collision between Gondwana and Armorican plates, and is characterized by a marked increase of the metamorphic degree from SW towards NE [1, 2]. From a tectono-magmatic point of view the hercynian chain of Sardinia has been divided classically in three main zones (fig. 1): external, thrust and axial zones from SW to NE [3, 4 and reference therein]. Palaeozoic sequences suffered a low-pressure high-temperature regional metamorphic event, with Barrovian grade from the external (greenschist facies) to the axial zone (amphibolite facies). Schematically, the overthrusted Armorican margin is represented by high-degree metamorphic complex outcropping in northern Sardinia, while the low- and medium-degree metamorphic complexes (Nappe Zone), outcropping in central and southern Sardinia, should represent the subducted Gondwana margin. The high-degree metamorphic basement and the Nappe zone are separated by a suture zone (the so called ÇPosada-Asinara LineÈ [5]), and is constituted of migmatites, amphibolites and eclogites. These latter are considered to be relicts of oceanic crust [5, 6].

The late-Hercynian granitoids During post-collisional phases, characterized by collapse and exhumation of nappe edifice, the whole metamorphic basement is injected by calcalkaline plutons mainly composed of monzogranites and granodiorites with minor amounts (< 5%) of gabbroic and tonalitic rock-types [7, 8]. Both meta and peraluminous granites crosscut the above- mentioned plutonic sequences and represent about 25% of the whole Sardinian batholith (about 6000 km2). In the tonalitic/granodioritic complexes the gabbroic rocks represent small intrusions earlier or contemporaneous with the associated tonalitic up to granodioritic rocks [8, 9, 190 F. SECCHI, M. LORRAI

10, 11]. On the contrary, in all the igneous sequences the leucogranitic intrusions represent the younger units. Both metaluminous (I-type) and peraluminous (S-type) granitoids coexist in some plutons. Most granitoids are I-type rocks and show high-K serial characters [8, 9]. The whole Sardinia batholith is crosscutted by a NS trending lamprophyric dike swarm showing calc-alkaline and transitional up to alkaline characters, emplaced from about 290-270 ± 10 Ma up to 230 ± 10 Ma respectively, as suggested by Ar-Ar and Rb-Sr radiometric ages [12]. Petrogenetical aspects of the Sardinian granitoids are still a matter of debate. Most of Authors proposed a mixing model of mafic and acid magmas to explain textural, petrochemical and isotopic features of the tonalitic/granodioritic association [8, 11, 13, 14, 15, 16]. However, for some plutons (i.e. Arburèse igneous complex), a model of crystal fractionation accompanied by crustal contamination (AFC) has been proposed [9, 17]. Most Authors agree that granitic and leucogranitic rocks originated by partial melting of continental crust [8, 18, 19, 20], even if in the Arburèse region (SW Sardinia) AFC processes have been proposed to interpretate the origin of peraluminous varieties [9, 17]. Finally, the petrogenic significance of some rock-associations (e. g. albite-rich pyroxene monzosyenites of the Sàrrabus area; fayalite-bearing granites of Quirra) is still obscure [21, 22].

Geological outlines of the Sàrrabus-Gerrei region The low-grade metamorphic basement of Sàrrabus-Gerrei region consists of allochtonous units made up of Cambrian to lower Carboniferous successions. The Hercynian structure of this region is dominated by several tectonic phases and synkinematic metamorphism under greenschist facies conditions (chlorite zone). Basement rocks are characterized by isoclinal folds recumbed south westward, subsequently refolded by several late-Hercynian phases [23, 24]. Basement rocks were intruded, at the end of Hercynian orogeny, by calcalkaline magmas forming the Sàrrabus igneous massif outcropping widely in the southernmost part of the region (figs. 1 and 2). The Sàrrabus igneous massif outcrops over an area of about 400 km2 and is a shallow pluton consisting of several intrusive sequences showing high-K calcalkaline affinity. Shallow conditions are constrained by low-grade regional metamorphism of the surrounding rocks as well as the development of narrow contact aureoles around the plutons with ubiquitous development of andalusite-cordierite hornfelses. Schematically, the Sàrrabus pluton is made up of several igneous units separated by sharp and discordant contacts, forming successive WNW-ESE and also NS trending belts (figs. 1 and 2). On the basis of field relationships, five igneous complexes have been recognized. In the following are given the main meso-to microscopic characters of the essential rock-units. From older to younger, the main complexes are: (a) Burcèi gabbrotonalitic unit («Burcèi type gabbrotonalite»; BU); (b) granodiorites with gabbrotonalitic septa (ÇCala Regina type granodioritesÇ; CR); (c) monzogranites (ÇMonte Nai type mozogranitesÇ; MN); SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 191

(d) leucogranites (ÇS. Priamo type leucogranitesÇ; SP); (e) fayalite-bearing biotite granite (ÇMonte Sette Fratelli type graniteÈ; SF). The whole Sàrrabus pluton is crosscutted by several late satellite dike to plug-like

Figure 2. Geological map of late-Hercynian granitoids from Sarrabus area (SE Sardinia, Italy). After BROTZU et al., in preparation, simplified. Legend: (1): Quaternary sedimentary deposits, Plio-Quaternary volcanics (Capo Ferrato area) and Oligo-Miocene sedimentary covers (Ussana Fm). 2-6: Late-Hercynian granitoids of Sàrrabus pluton. (2): Fayalite- bearing biotite granites (Sette Fratelli type granite); (3): biotite leucogranites (S. Priamo type leucogranites); (4): monzogranitic granodiorites and monzogranites (Monte Nai type monzogranite); (5): hornblende biotite granodiorites (a) with gabbrotonalitic masses (b) (Cala Regina type granodiorite); (6): two-pyroxene gabbrotonalites of Burcèi area (Burcèi type gabbrotonalite); (7): Palaeozoic metamorphic Basement. (8): main faults. Note that in order to simplify late-hercynian dike swarm was not plotted. 192 F. SECCHI, M. LORRAI bodies mainly of granitic to monzosyenitic composition and dike swarms ranging in composition from monzosyenites, basalt to rhyolite.

Field relationships between igneous masses and emplacement age of Sàrrabus pluton The emplacement age of the plutonic rocks is regionally constrained by the discordant contacts with the Cambrian-lower Carboniferous basement, well exposed along the northern boundary of the igneous pluton. Furthermore, intrusives do not show any kind of post-emplacement deformations. The emplacement of the southern Sàrrabus granitoids occurred after the main hercynian orogenic events; several published Rb-Sr regression lines confirm the constraints imposed by regional geology. Particularly undistinguishable values of 311 ± 9 Ma and 301 ± 3 Ma are obtained for gabbrotonalitic rocks of Burcèi area and southern granodiorites respectively, close to 305 Ma between lower and upper Carboniferous [10, 25]. On the basis of field relationships, the Sàrrabus igneous pluton shows a change of composition with time; in the order (from older to younger) Burcèi gabbrotonalitic unit → Cala Regina granodiorites (with gabbrotonalitic septa) → Monte Nai type monzogranites and S. Priamo type leucogranites → NS trending Sette Fratelli fayalite-bearing granites. The following geological picture is based essentially on the recent geological survey carried out by a research group including P. BROTZU (University of Naples), E. CALLEGARI, (University of Turin) and FRANCESCO SECCHI (University of Sassari).

Burcèi gabbrotonalitic body («Burcèi type gabbrotonalite»; BU) Geological and petrographical features of this body have been outlined by Brotzu [10]. Schematically, it is a composite sill-like body located in the northernmost part of the Sàrrabus pluton, at the contact with Palaeozoic basement. It is constituted of two separate injections of magma ranging in composition from two-pyroxene biotite gabbrotonalites to orthopyroxene-bearing granodiorites. The rocks of the two different intrusions are quite homogeneous granular to porphyritic rocks; gabbrotonalites show relatively fine- grain size, interpreted as an evidence of fast-cooling of magmas intruded in a predominant molten condition [10]. Along the contacts with late leucogranitic igneous body, Burcèi intrusive rocks are dismembered into many angular blocks (up to 1 m). The Burcèi gabbrotonalitic suite seems to be governed by crystal/liquid fractionation processes of a mantle/derived parental magma geochemically comparable to some mafic dikes occurring in the Sàrrabus pluton [10].

Granodiorites (ÇCala Regina type granodioriteÈ; CR) Granodiorites constitute a number of discrete intrusions separate by sharp contacts widespread in the southern side of Sàrrabus pluton. The granodiorites outcropping in the southernmost area are enriched in dark enclaves and display a moderate magmatic foliation. These rocks are equigranular medium-grained rocks and, under the microscope, SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 193 they are better defined (according to IUGS’s nomenclature [26]) as hornblende biotite granodiorites, while the granodiorites occurring in the northernmost portions are essentially biotite granodiorites straddling to the boundary with the monzogranites. Along the southern coastline, granodiorites contact with small (about 0.5 km2 sized) irregular or roughly elliptical masses made up of several terms of gabbrotonalitic composition. These gabbrotonalitic masses represent likely early intrusions emplaced into the metamorphic basement and dismembered by successive granodiorites (STOP 2; Plate 1a) as testified by the occurrence of sub-angular blocks (up to 2 m; Plate 1b) of gabbroic rocks dispersed in the hosting granodiorite. The gabbro tonalitic masses are constituted of several sub-units of gabbroic, tonalitic (up to granodioritic) composition; in the field they are characterized by pillow-like structures (Solànas; Plate 2a and 2b) suggesting that the two terms are substantially coeval. Gabbrotonalitic rocks are characterized by plagioclase, amphibole (likely formed at the expense of former pyroxene) and, locally, biotite with a sialic/mafic ratio of about 1/ 1. Amphibole pegmatites occur in all the outcrops; in particular, in the Scala Carbonara outcrop they reach about 50 cm in size. The grain size increases from the outer to the inner part of the bodies, the ratio plagioclase/mafic remaining unchanged (Scala Carbonara and Campulongu). The largest outcrops of tonalites and granodiorites are located between Baccu Mandara and Solànas; they are inhomogeneous bodies characterized by sudden transitions from tonalitic to granodioritic facies. These rocks show medium to medium-coarse granular textures and moderate to strong magmatic foliation revealed by the strong flattening of the dark inclusions of quartz gabbroic composition (Plate 3a).

Monzogranites and leucogranites (Monte Nai type monzogranites; MN) Monzogranites outcrop in the central part of the pluton. These rocks are coarse- grained, light grey, biotite rocks. Dark enclaves are rare. These rocks are quite often crosscut by sub-vertical aplogranitic dikes.

Leucogranites (ÇS. Priamo type leucogranitesÇ; SP) Leucogranites outcrop widely in the northern side of the pluton. These rocks are coarse-grained, pinkish, biotite rocks. Dark enclaves are very rare, while biotite-bearing pegmatitic pockets are quite commonly observed. These rocks are quite often crosscut by sub-vertical aplogranitic dikes; mafic dikes are quite rare.

Fayalite-bearing biotite amphibole monzogranite (ÇSette Fratelli type graniteÈ; SF) Light greysh leucocratic medium to coarse-grained rocks. Dark enclaves are quite rare. These granites are characterized by the common occurrence of Fe-hastingsitic amphibole and Fe-biotite as mafic phases, as well as fluorite in accessory amounts. Some typical pseudomorphs suggest the previous existence of subordinate fayalite. The SF granites are located in the central part of the Sàrrabus igneous massif; they crosscut with 194 F. SECCHI, M. LORRAI vertical contacts the surrounding monzogranites. All the up mentionated plutonic units are crosscutted by an early monzosyenitic dyke complex and successively by a quite voluminous dike swarm of (mostly penecontemporaneous) basaltic to aplogranitic magmas.

Monzosyenites As outlined by Pirinu et al. [21], the Sàrrabus igneous complex is crosscut by EW trending dikes and stocks of monzonitic to syenitic composition, mainly hosted into granodiorites in a narrow area close to the boundary with monzogranites. Monzosyenitic rocks form discontinuous outcrops showing sharp and discordant contact field relationships with surroundings. In the field monzosyenitic rocks are coarse-grained leucocratic rocks with a spotted texture due to the clustering of diopsidic pyroxene and amphibole. These rocks are often crosscut by hololeucocratic veins of quartz syenitic composition. Under the microscope monzosyenitic rocks show the effects of Na-metasomatism documented by the occurrence of late albitite veins and the large development of secondary albite. According to PIRINU et al. [21], the documented petrographical and petrochemical contrasts between monzosyenites and their host granodiorites indicate that monzosyenites represent a rock-series genetically indipendent from the granodioritic magmas.

Mafic and acidic dike swarm Voluminous mafic and acidic dike swarms are mainly oriented NNW-SSE, and subordinately NS and NE-SW. The acidic hypabissal facies are mainly porpyritic aplogranites. The mafic dykes, generally altered, are mainly distributed into granodiorites (Plate 3a) bodies and consist of medium to coarse-grained gabbroic facies in the inner part gradually changing to aphyric and porphyritic border facies. Locally, composite bodies occur, probably due to autointrusion processes (Scala Carbonara, Cabu Oi; Plate 3b). Plagioclase and amphibole represent the most abundant mineralogical phases in hypabissal rocks; pyroxene, generally subordinated, sometimes represents the only mafic phase. Acidic dikes are the most widespread: particularly in Solànas, Villasimìus and Cala Pira areas they make up wide swarms with NNW-SSE direction. Their attitude is constantly vertical or sub-vertical and their thickness ranges from 1 to 15 m; locally (Cala Pira), acidic dykes 30 m thick are found. The structures of acidic dykes vary from strongly porphyritic to afiric, with plagioclase, quartz and alkaline feldspar as main mineralogical phases. Biotite represents the only mafic mineral.

MINING ACTIVITIES AND ENVIRONMENTAL ISSUES Ore deposits in the Sàrrabus-Gerrei region For the economic and scientific importance of its ore deposits exploited for Ag, Pb, Zn, Sb, As, Mo, F and Ba, the Sàrrabus-Gerrei region represented the second mining district of Sardinia. SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 195

Since the second half of the XIX century major mining works were concentrated along a complex of silver veins hosted in Silurian-Devonian metasediments, extended for more than 30 km, the so called Çargentiferous vein of SarrabusÈ. The paragenesis of this group of deposits is constituted of quartz, calcite, fluorite and barite as gangue minerals, associated with Pb-Zn-Ag sulphides and native Ag. These are interpreted as syngenetic stratabound deposits, often remobilized and redeposited in late-fracture systems, sometimes in composite association with mafic dikes, during late-Hercynian magmatic and hydrothermal activity [27]. Other important ore deposits in the area were those of Baccu Locci, mined for lead and arsenic (1873-1965), Su Suergiu and Corti Rosas (1882-1968), both mined for antimony. In particular, the Su Suergiu mine was one of the biggest antimony mines in Italy. At Su Suergiu the antimony deposit is hosted in the tectonic breccia of the shear zone of the Villasalto Overthrust, and contains lenticular pieces and bodies of stibnite in a matrix of Silurian black shales, with scattered grains and small layers dominated by pyrite [28]. There are also thin layers and veins of scheelite associated with calcite. The tectonic breccia in the area of Villasalto reaches an exceptional thickness (fig. 3), over one hundred metres, and is ore-bearing over a stretch of almost 2 km. All the ore bodies have the shape of lenses of various sizes, flattened more or less parallel to the tectonic contact and elongated in EW direction. The origin of the ore deposit of Villasalto is still controversial, being considered as sedimentary, extrusive-sedimentary or magmatic-hydrothermal. The more recent studies interpret it as the result of a primary high-grade epigenetic deposit of magmatic- hydrothermal origin, prior or contemporary with the overthrust [28]. However, many

Figure 3. Schematic geological cross-sections of Palaeozoic basement of Villasalto area [29]. Genn’Argiolas Unit: COr: ÇArenarie di San Vito FmÈ (Cambrian-lower Ordovician); br: polygenic tectonic breccias constituted mainly of Silurian black slates. Gerrei Unit: Ca: metaconglomerates (lower Carboniferous); D: metalimestones (upper Devonian-lower Carboniferous); SD: marly and carbonaceous slates with interbeddings of metalimestones (lower-middle Devonian), fossiliferous slates, metalimestones and black quartzites (Silurian); Or: fossiliferous metasiltstones, metarkoses and metasandstones (upper Ordovician); π: metarhyolites (Ordovician); α: metamorphic products of reworked volcanites and intermediate to acid metavolcanites (Ordovician). 196 F. SECCHI, M. LORRAI questions are still unsolved because the tectonic shattering and transport within the tectonic breccia have obliterated any evidence about its origin. Mining activities have played an essential role in the economic, social and cultural growth of the area, which is included in the proposed Geominerary Park of Sardinia, sponsored by UNESCO. Su Suergiu is planned to be an important site for the future park.

The mine and the foundry of Su Suergiu Exploitation at the Su Suergiu mine started in 1858, but only after 1882 mining works began quite important. Antimony minerals were extracted from carbonaceous slates, subjected to handpicking and mechanical grading prior to be melted in furnaces supplied with wood. After this pioneering period the deposit was intensively mined, especially during war periods, due to the use of antimony in the war industry. In 1905 the foundry was upgraded with some structures including a particular system for the production of antimony oxide and a new smelter for its transformation in metallic antimony. In the smelter at the temperature of 550°C the antimony oxide was mixed with charcoal, necessary for the reduction process → according to the reaction Sb2O3 + 3C 2Sb + 3CO, and with the introduction of sodium carbonate for a better fluidity of the melted product [30]. In the 1960’s the foundry increased the activity, due to a particular rotary furnace and to the supply of antimony minerals from other mines in Italy (Manciano, Tuscany), Turkey and China. The mine and the foundry stopped all the activities in 1987; the production of the foundry up to 1971 is summarized in tab. 1. At the Su Suergiu mine several reclamation activities, such as restoration works of buildings, roads and adits, and the protection of open spaces are in progress. However, nothing has been made for the remediation of an invisible but certainly more harmful phenomenon: the mobilization of toxic elements in the environment. The dispersion of contaminants can be referred both to natural processes (due to the erosion of mineralised rocks and water-rock interaction in the area) and especially to the past mining and smelting activities. In particular, water is the main pathway for contaminant transport, through the mechanical erosion of the mine dumps and foundry wastes, as well as transport of contaminants

Table 1. Summary of the productions of the foundry [33].

Product Period Tonnes

Sb metallic 1907-1971 15853 Sb oxide 1884-1971 3918 Italox* 1931-1953 1152 Melted Sb sulphides 1882-1971 9529

* A particular type of antimony oxide with a candid white colour, used for pigments. SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 197 in solution. Water-rock interaction occurs into the system of galleries and dumps of mine wastes, as well as along the river downstream of the mine. Water samples taken from rivers and springs in the whole area show a severe arsenic and antimony contamination.

Hydrogeochemistry in the Su Suergiu area Abandoned mines are often sites with a high contamination risk, especially for the

Figure 4. Location of water samples taken in the Su Suergiu area and their relationships with dumps of waste materials. The water sampling point just downstream the main waste dump has been sampled twice. 198 F. SECCHI, M. LORRAI

Figure 5. ÇPiper dia- gramÈ for water samples from Su Suergiu area (Villasalto). Symbols: streams (triangles); springs (squares); drainage from adit (circle). mobilization of heavy metals and other contaminants. Such phenomenon has been investigated in several areas of Sardinia [31, 32, 33, 34, 35, 36, 37, 38, 39, 40] and also in the Sarrabus-Gerrei region [40, 41, 42, 43, 44], pointing out the severe impact of the past mining activities on the water quality. In order to evaluate the water characteristics and the effects of past mining activities on water quality at Su Suergiu, samples were taken from rivers, springs and adits in the area (fig. 4). The lithologic features and the presence of the ore deposit, as well as the presence of wastes derived from the past mining and smelting activities are well reflected in the geochemical characters of the water samples. Underground and surface waters of the area, non-interacting with the mineralized rocks and with the mine-wastes, have a calcium-bicarbonate character, according to the presence of limestones constituting the main aquifers, and subordinately a sodium(calcium)- chloride(bicarbonate) character. The salinity in these waters ranges from 0.2 to 0.8 g/l, and the pH varies between 7 and 8. No contaminants were detected. Following interaction with the mineralization, water (drainage from an abandoned adit) shows a calcium(magnesium)-sulphate character, and is enriched in antimony (0.35 mg/l). The salinity of this groundwater is about 3 g/l and the pH is 7.8. Following interaction with the mine wastes, surface waters acquire a sodium- sulphate(bicarbonate) character, and are heavily polluted by arsenic and antimony (up to 80 mg/l and 11 mg/l, respectively). The pH is up to 9.6 and the salinity up to 4.5 g/l. The change of water character from bicarbonate to sulphate is related to the oxidation of sulphide minerals; the high amount of sodium is due to the sodium-carbonate used in the SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 199 smelting process and accumulated in the waste dumps. During the dry season, the stream flowing through the mine area is almost dry, and

Figure 6. Plots showing TDS (total dissolved solids) vs. main cations and anions for water samples from Su Suergiu area (Villasalto). Note the major contribution of Na, SO4, and HCO3 to the salinity. The source of sodium is thought to be the sodium carbonate used in the metallurgical process in the foundry, and now present in the waste dumps. The shape of the symbols refers to the sample type: streams (triangles), springs (squares) and drainage from adit (circle); the fillings of the symbols refers to the chemical species.

Figure 7. Plots showing dissolved contents of As and Sb vs. pH, TDS, Na and SO4 in waters from Su Suergiu area (Villasalto). High values of As and Sb are linked with high pH, high salinity and high sodium and sulphate contents. The shape of the symbols refers to the sample type: streams (triangles), springs (squares) and drainage from adit (circle); the fillings of the symbols refers to the chemical species. 200 F. SECCHI, M. LORRAI

Figure 8. Plots showing pH vs. main cations and main anions in waters from Su Suergiu area (Villasalto). High values of pH are linked with high sodium, carbonate and sulphate dissolved contents. The shape of the symbols refers to the sample type: streams (triangles), springs (squares) and drainage from adit (circle); the fillings of the symbols refers to the chemical species. only in its middle and lower course can be observed a small flow, due to the inflow of the Rio Ratti tributary. The Rio Ratti water has a calcium+sodium - bicarbonate+chloride character, TDS = 827 mg/l and pH=8. Downstream of the confluence (fig. 4), the dissolved content of arsenic changes from 15 to 145 µg/l over a few hundred metres, while antimony changes from 4 to 1245 µg/l. This increase in dissolved arsenic and antimony contents is due to the interaction between water and contaminated stream sediments. The contamination of stream sediments has been well observed during studies carried out in the area for mineral prospecting and environmental purposes [40, 45]. Arsenic and antimony in stream sediments can be referred both to the natural erosion of mineralized rocks of the district and to the erosion, transport and deposition of wastes resulting from mining and smelting activities. The hydrogeochemical study has pointed out that the main sources of contamination in the area are: 1. Waste dumps; 2. Contaminated stream sediments; 3. Drainage from abandoned adits. Due to the long stretch of the river involved in the contamination process it is very difficult to plan remediation strategies for the stream sediments, but is relatively easy and economically feasible the capping of the waste dumps, with the aim of preventing water circulation into the wastes. In alternative, the treatment of the contaminated water flowing downstream of the mine can be considered.

DESCRIPTION OF STOPS Stop 1. Solànas (loc. porto Murroni) Cala Regina type granodiorites and geological aspects of mafic dike swarm. The Cagliari-Villasìmius main road crosscuts the granodiorites and their mafic septa, that SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 201

Figure 9 202 F. SECCHI, M. LORRAI

Plate 1a. Contact field relationships between Solànas gabbroic rocks and granodiorites. Small dikes of granodiorites (light portions) crosscut the gabbroic rocks. Loc. Cuccuru de Portu Perdosu, northwestward the Solànas village.

Plate 1b. Contact field relationships between Solànas gabbroic rocks and granodiorites. Western contact of Scala Carbonara mass: sub-angular blocks of gabbroic rocks dispersed into granodiorites. Loc. Scala Carbonara, along the Cagliari-Villasimìus main road. SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 203

Plate 2a. Gabbro-tonalitic interaction in the igneous masses of Sàrrabus igneous massif. Pillow-like structures due to the dismembering of hornblende-biotite quartz gabbroic rocks (dark enclaves) into hornblende tonalites. Loc. Solànas.

Plate 2b. Particular of the previous 204 F. SECCHI, M. LORRAI

Plate 3a. Field aspects of Sàrrabus intrusives. Magmatic foliation of hornblende quartz diorites evidenced by the strong elongation of dark gabbroic enclaves. Loc. Porto Murroni, westward the Solànas village.

Plate 3b. Contact field relationships between Torre de su Fenugu gabbroic rocks and granodiorites. Synmagmatic dikes of gabbroic composition dispersed into granodiorites. Loc. Torre de su Fenugu, along the coastline. SOME GEOLOGICAL AND ENVIRONMENTAL ASPECTS OF THE SARRABUS-GERREI REGION… 205 develop pillow-like structures (Plate 2a and b). Contact field relationships between gabbrotonalitic rocks and hosting granodiorites (Plate 1a).

Stop 2. Torre de su Fenugu Cala Regina type granodiorites. Contact field relationships between gabbrotonalitic rocks and granodiorites. Pillow-like structures of gabbroic rocks in granodiorites, interpreted as the result of synmagmatic injections (Plate 3b).

Stop 3. Monte Sette Fratelli (Loc. Maidopis) The Monte Sette Fratelli intrusion represents one of the youngest intrusive units of the Sàrrabus igneous massif. This unit is constituted of light greysh equigranular monzogranites characterized by the occurrence of Fe-hastingsite, Fe-biotite and accidental fayalite as mafic constituents, as well as typical fluorite as accessory phase. At large scale, it can be observed the contact field relationships between this unit and the granodiorites of eastern zone, as well as the contact with the metamorphic basement.

Stop 4. Villasalto (loc. Su Suergiu) Geological setting and characteristics of the antimony deposit. View of the abandoned mine area which is an example of industrial archaeology, and visit to the museum of the mine organised into the restored buildings. Environmental problems linked to a severe water and sediment contamination by arsenic and antimony from waste dumps.

ACKNOWLEDGEMENTS We are grateful to Mr Silvestro Frau and Mr Mario Garau, Councillor and Mayor of Villasalto respectively, for granting the access to Su Suergiu mine, and to Dr Isabella Quartu, Dr. Tiziana Serrau, Dr Sergio Utzeri and Dr Giuseppe Melis for their precious collaboration. The authors are indebted with Dr Gabriella Cossu for her valuable contribution during the field trip and helpful discussions.

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